Drills with indexable cutting inserts, comprising one center insert and one peripheral insert, are previously known from the patent U.S. Pat. No. 4,149,821 and others, and from this it is known that suitable choice of cutting angles and the location of the inserts relative to each other will make it possible to balance the cutting forces so that there is no radial force component when all inserts are cutting.
When this type of drill is used for drilling of through holes, the center insert will cease cutting when the peripheral insert is still machining the last part of the hole wall. The cutting force on the peripheral insert will then no more be balanced, and since it usually has an inward radial component, the drill may be deflected inwards and the diameter of the last part of the hole be diminished. This is especially notable for drills with wide chip grooves, since their bending stiffness is low in the direction of the grooves.
It has been suggested in the patent DE 35 45 586, which concerns a drill where there is a large radial force component even with both inserts cutting, that the depth of the chip grooves should become smaller and the pitch angle steeper with increasing distance from the tip. This would increase the bending stiffness as the bending moment increases. It was also suggested that each chip groove should circle preferably 250-270 degrees of the drill, which would ensure that the bending moment when both inserts are cutting would tend to bend the drill in its stiffest direction both at the end of the grooves and at a midway point where the groove depth is still great. This does not, however, indicate that the deflection at the end of a through hole is minimized or has a specified direction.
The present invention concerns an improved drill with indexable cutting inserts, comprising one center insert and one peripheral insert, where the improvement consists in giving the chip grooves a spiral shape circling the drill for such an angle that the cutting force on the peripheral insert will cause a minimal radial deflection, allowing somewhat larger tangential deflection.